Sludge is a mixture of dense material that is collected from raw sewage during primary treatment, and secondary biomass that rapidly grows during the secondary and tertiary treatment steps in a conventional wastewater treatment plant. According to regulations set by the U.S. Environmental Protection Agency (EPA), the sludge produced by any single wastewater plant must be subjected to a treatment strategy that will result in a 38% reduction of the total volatile suspended solids (VSS) and a final concentration of fecal coliform that is less than 2×106 colony forming units. After meeting these standards, the treated sludge is considered Class B biosolids and may be disposed of in a landfill, or land applied to a restricted site as defined by 40 CFR Part 503 of the Clean Water Act.
Sludge treatment strategies often exploit the activity of microorganisms to remove organic contaminants from waste streams. Strategies include aerobic and anaerobic methods for wastewater and sludge treatment. However, such conventional strategies suffer from numerous disadvantages. Aerobic methods, for example, require a significant amount of energy input to mix and to aerate reactor contents. Such sludge treatment methods also result in large volumes of secondary biomass that must be treated, leading to extra energy cost for treatment and for disposal.
Anaerobic sludge digestion processes enable a limited amount of energy recovery through, for example, methanogenesis and co-generation. However, energy production through such processes is inefficient and excess methane often must be burned as a waste gas. Anaerobic digesters also require a long residence time and multiple reactors must be employed to treat the large sludge volumes produced in cities. As a result, such digesters require much higher levels of energy than they are able to produce, as well as a large land area for operation. Anaerobic sludge digestion also produces a large amount of secondary biomass and recalcitrant solid waste products, requiring additional treatment and disposal cost.
Microbial fuel cells (MFCs) offer the potential to employ microorganisms to convert the energy stored in organic carbon compounds (waste) into electricity. The flow of electrons through the MFC system results in accelerated primary sludge reduction, reduced volumes of secondary sludge and direct electrical power generation. The catalytic activity of an MFC is generated by microbes, generally, bacteria, that attach to the conductive surfaces of electrodes and form electrochemically active biofilms. Microbes within the biofilm at the anode enzymatically extract electrons from organic components in the sludge, wastewater or other liquid input and transfer the electrons to the electrode. The microbes must perform the electron transfer to the electrode surface to maintain biological functions, in other words, the microbes “breathe” the electrode surface to live. Because MFC systems are designed to immediately move the electrical energy away from the microbes through electrical current generation, the microbes are unable to use the energy for growing and for building biomass. Furthermore, the movement of energy away from the microbes also accelerates microbial metabolism and increases primary sludge reduction rates.
Completion of the reactions in existing MFC devices takes place in physically separate, but electrically linked, compartments with different bacterial biofilms. The cathode is used as a source of energy during the reduction of oxygen or other oxidant, such as a nitrite, a sulfate or a heavy metal. The cathode is submerged in a liquid and therefore bacterial growth on the cathode is limited by the energy source being delivered across the circuit and therefore, biomass production is reduced, relative to traditional aerobic treatment systems. Additionally, the production of new water results from the biologically catalyzed oxygen reduction reaction with the cathode. For example, one new molecule of water can be produced for every four electrons and two protons that cross from the anode compartment to the cathode compartment during MFC operation, for example, when oxygen is the oxidant. The production of water is biologically catalyzed and can be optimized based on how the MFC system is operated.
The products of an MFC system include: 1) treated non-potable water (to secondary levels) or potable water and carbon dioxide from the anode; 2) a new source of water evolving from the cathode, for example, when oxygen is included as an oxidant; and 3) electricity as a result of the bioelectrochemical reactions in both compartments.
Research has shown that MFC systems operating with sludge as a fuel source are able to degrade between 40% to 80% of the initial organic content within twelve hours of residence time (Logan, B. E. (2005) Waste Science and Technology 152:31-37; Scott, K. and C. Murano (2007) Journal of Chemical Technology & Biotechnology 82: 92-100; Mohan, S. V. et. al., (2008) Biosensors and Bioelectronics 23: 1326-1332). However, the work was conducted in laboratories, using reactors holding 30 to 500 milliliters of wastewater.
MFC based systems that can effectively treat wastewater on an industrial scale are needed.